Well tubing cutting device



y 1965 e. a. CHRISTOPHER 3,192,857

WELL TUBING CUTTING DEVICE Filed Oct. 8, 1962 I 2 Sheets-Sheet 1 3 I 3 INVENTOR 1 38 Glenn B. Chnsfopher 36 as 5/ 53 F|g.l

ATTORNEY y 1965 s. B. CHRISTOPHER 3,192,857

WELL TUBING CUTTING DEVICE v Filed Oct. 8, 1962 2 Sheets-Sheet 2 34 l k j INVENTOR Glenn B. Christopher BY 62 (Q ATTORNEY Unitcd States Patent 3,12,857 WELL TUfdiNG CUTTING DEVICE Glenn 3. Christopher, Fort Worth, Tex., assignor to let Research G'Jenter, Inc, Ariington, Tex, a corporation of Texas Filed Oct. 8, 1962, Ser. No. 229,151

- 4 fiiaiins. (Ci. lull-) This invention relates to a device for severing well tubing and the like by means of an annular shaped explosive charge.

Permanent type well completion techniques often require that the lower end of a tubing string be equipped with a guide shoe, bul-l plug, valve or other equipment which closes or restricts the end of the tubing. To complete the well using a through-tubing perforating gun, it is necessary that the obstruction be removed from the lower end of the tubing.

Other occasions often arise where it is desirable to out well tubing in place, such as where the tubing has become stuck in the well, cannot be released from a packer, or the casing has collapsed on the tubing.

A large part of the tubing cutting operations conducted in recent years has utilized devices that may be run into the well on the end of an electric wireline. A pressureresistant housing is used which contains an annular shaped explosive charge that, upon detonation, produces a planar cutting jet that is directed radially outwardly from the housing to cut through the tubing walls. The explosive load is carefully sized to insure that the tubing is cut through without causing any damage to the surrounding casing. One commercially successful form of a tubing cutting device which minimizes the flaring or splitting and outward bending of the cut tubing is disclosed in my copending application for United States Letters Patent, Serial No. 766,343, filed October 9, 1958, for Apparatus for Cutting Oil Well Tubing and the Like, now Patent No. 3,057,295. The invention disclosed in this application is directed to improvements in the tubing cutter disclosed in my foregoing copending application.

In conducting tubing cutting operations, it is sometimes necessary that the retrievable portion of the tubing cutter housing pass through landing nipples installed in the tubing having a smaller inside diameter than the tubing. The slight flaring of the retriveable upper portion of the housing produced :by the planar jet at the cut end of the housing has caused difficulty in retrieving the housing through close fitting nipples. Also, the severed, non-retrievable, lower end portion of the housing of the tubing cutter described in my foregoing copending application remains intact in the well and may bridge the opening in the lower portion of the cut tubing and interfere with subsequent down-hole work.

Therefore, it is an object of this invention to provide a tubing cutting device having a housing with a retrievable portion that will easily pass through close fitting openings after the device has been fired.

Another object of this invention is to provide a tubing cutting device that has a housing, a portion of which is retrievable, that is adapted to shear transversely with a minimum of flaring of the retrievable portion of the housing.

A further object of this invention is to provide a tubing cutting device with a housing having a non-retrievable lower portion adapted to be broken into several pieces by detonation of the shaped explosive charge.

The foregoing objects of the invention are achieved, in general, in a device for severing well tubing and the like which includes a short cylindrical, pressure-resistant housing including a hollow body portion. A separate end plug may sealingly close the lower end of the body. The body portion has a thick annular side wall and a thick 7 being indicated in FIG. l.

top wall, which may be integral therewith. The side Wall provides a first horizontal annular groove, preferably located in theexterior surface of the lower portion thereof. The side wall provides a second horizontal annular groove in the interior surface of the lower portion thereof immediately below the first annular groove. The side wall has a thin section bounding the outer periphery of the second groove. The body portion below the first annular groove has an outside diameter that is greater than the body portion above the first annular groove. A generally cylindrical, shaped explosive charge, having a main charge or" explosive material with an outwardly-opening, annular, lined cavity in the periphery thereof, is mounted facing the .thin section of the side wall. The charge is adapted upon firing to producea radially expanding planar cutting jet adapted to sever the side wall at the thin secti-on.

Means are provided to maintain the cartridge in a fixed position with the cavity facing the thin section. The top wall and the cartridge provide openings therethrough adapted to receive a portion of a detonating fuse. The housing and the cartridge define therebetween a substantially unobstructed space above the cartridge providing an expansion chamber for containing upwardly acting forces produced upon detona-tionqof the cartridge. The body is adapted to be cleanly severed transversely at the first annular groove by detonation of the cartridge. The side walls of the body above the first annular groove are of sufiicient thickness and strength to remain intact and to be substantially undistorted after the cartridge isfircd.

In accordance with another aspect of the invention, the separate endplug is provided with an axial bore and an annular groove surrounding the bore in the top surface thereof.

'The'foregoing objects and aims, and other objects and attendant advantages, will be set forth in detail in the following description of the invention taken in connection with the drawings.

In the drawings:

FIG. 1 is an axial sectional view of the lower portion of a well tool assembly including an exemplary tubing cutting device embodying the principles of the invention;

FIG. 2 is an enlarged sectional view along line 2-2 of FIG. 1; e

FIG. 3 is an enlarged sectional view along line 3-3 of FIG. 1;

FIG. 4 is an enlarged sectional view along line 4-4 of FIG. 1; and 7 FIG. 5 is an enlarged sectional view of the retrievable body portion of the tubing cutting device of FIG. 1 after the shaped charge cartridge has been fired.

Referring to FIG. 1, the assembly herein shown has a cutter head generally designated by the numeral 10. The cutter head is threadedly connected at its upper end to a firing headl l. The latter is secured to an adapter sub 12 which is attached to a conventional casing collar locator 13, only the lower end of the latter. component In accordance with common practice, the casing collar locat-or is suitably connected to a rope socket (not shown) to which is attached the usual conductor cable (not shown) by which the apparatus is lowered into the well tubing to be severed and by which firing current is delivered to the apparatus.

The adapter sub 12 has an axially depending, cy-lin drical projection 14 fitted snugly within a cylindrical bore 15 formed in the upper end of the firing head.

Allenhead cap screws 1615, threaded into the depending projection 14 and passing throughholes 17'17 in the firing head, securely fasten the firing head to the adapter sub. O-rings 18-18 seal the firing head to the projection 14 and prevent ingress of well (fluids into the bore 15. a r

The firing head has an axial bore .19 of reduced diam- 3 eter communicating with the bottom of the bore 15. Another bore of still further reduced diameter extends axially downwardly from the bore 19 to communicate with a recess 21 extending axially upwardly from the bottomof the firing head.

An electrical contact member 22 is insulatedly supported by the projection 14 at its lower end and is electrically connected by insulated conductors (not shown) extending upwardly through the assembly to the wire of the conductor cable. An insulating sleeve 23, of resilient material such as rubber, is inserted in the bore 19. The sleeve frictionally supports a metallic spring 24 that makes cont-act at its upper end with the contact member 22 and is electrically connected at its lower end to the terminal wire 25 of an electric blasting cap 26. The other terminal wire 27 of the blasting cap is grounded to the assembly by having its free end looped under the bottom of the insulating sleeve 23 and pressed into contact with the wall of the bore 19. It will be noted that the blasting cap is in part supported in the assembly by the walls of the bore 20.

The recess 21 of the firing head has internal threads 28 at its lower end which mate with the external threads 29 provided on the cylindrical boss 30 of the cutter head 10. An O-ring 31 seals the boss within the recess 21 of the firing head to prevent intrusion of well fluids.

The cutter head 10 has a top wall 32 and an annular cylindrical side wall 33. The upper portion 33a of the side wall 33 has a diameter slightly smaller than the lower portion 33b of the side wall which is below a generally V-shaped first annular groove 34 provided in the exterior of the side Wall 33. The open lower end of the housing is closed by an end plug 35 having a cylindrical portion 36 that makes a sliding fit inside the lower end of the housing. An O-ring 37 is received in a. groove provided in the periphery of the cylindrical portion of the plug to provide a seal with the interior of the lower portion 33b of the side wall. Drive screws 38 received in aligned openings provided in the edge of the plug 35 and the bottom of the lower portion 33b of the side wall attach the plug to the side wall.

A shaped explosive charge cartridge 39 is mounted within the chamber 40 of the cutter head. A-sdescribed in my aforementioned copending application, the chamber 40 provides an expansion chamber into which the upwardly acting gases generated by the detonating cartridge may expand and be confined. The forces so generated are absorbed and contained by the chamber 40 and the heavy side wall 33 to prevent the energy from acting on the cut tubing end to flare or split the bottom end of the cut tubing. The cartridge includes identical lower and upper end plates 41 and 42 which are circular in horizontal cross-section. The outer edge of each plate is beveled as shown at 43 in FIGS. 1 and 2. Each end plate-hasan inwardly-projecting cylindrical portion or boss 44 providing a thickened central section for the end plate. Each end plate provides an axial opening 45 to receivea portion of a detonating fuse 46, such as Primacord.. Themain shaped'explosive charge is formed from lower and ,upper annular elements designated 47 and 48, respectively. Each element is made of a detonating explosive material, such as compressed waxed RDX for example. The outer peripheral face of each element is in: wardly inclined and is lined with an inert liner section 49. Theinner edgesof the liner sections abut at the bottom of the generally. V-shaped groove formed by the outwardly flaring faces of the two explosive elements. The car tridge 39 is assembled by gluing together two half-sections. Each half-section consists of an end plate, a cornpressedexplosive element, and a liner section which has previously been assembled into a unitary assembly. A top view of such a half-section of the cartridge 39 is shown in FIG. 4. The explosive elements provide axial openings aligned with the openings in the end plates .to receive the detonating fuse 46.

The inside diameter of the chamber 40 adjacent the lower portion 33b of the Wall 33 is enlarged slightly by counterboring to provide a shoulder 50 that is opposite the groove 34 in the exterior of the side wall 33. The cartridge 39 is received in the counterbored portion of the cutter head and is seated against the shoulder 50. A cylindrical diaphragm spring 51, positioned between the top surface of end plug 35 and the bot-tom surface of the lower end plate 41, maintains the cartridge 39 firmly seated against the shoulder 59. Referring to FIG. 3, it is seen that the spring 51 has a number of cut-out, upturned spring fingers 52 which engage the bottom of the lower end plate to maintain the cartridge in a fixed position. The end plug 35 has a deep annular groove 53 in the top surface thereof which defines a cylindrical center portion 54 of the plug. This center portion 54 has an axial bore 55 which receives the end of the detonating fuse 46. The bore extends to about the same depth in the plug as does the groove 53. A fuse tube 58 extends from the top wall 32 through chamber 40 to the top of the upper end plate to guide the detonating fuse through the openings 45 in the end plates into the bore 55.

Referring now to FIG. 1, the lower portion 33b of the 7 side wall provides a second annular, generally U-shaped groove 56 in the interior surface thereof immediately below the first exterior annular groove 34. The groove 56 is opposite the annular lined cavity of the cartridge 39 and is symmetrical about a horizontal plane passing medially through the cartridge. The bottom of the groove defines a thin wall portion 57 in the lower section 33b of the side wall. This thin wall portion of the cutter head is made only thick enough to withstand hydrostatic pressures expected to be encountered in a well bore so that it may be readily cut by the planar jet with a minimum of energy absorption.

The cutter head 10 can be made of any material hav-. ing suflicient strength whereby the entire assembly will resist the amount of hydrostatic pressure expected to be encountered in a well bore and which material will provide sufficient strength for the upper wall section 33a and top wall 32 to remain intact and to be substantially undistorted by the detonation of the shaped charge explosive cartridge. The heads can be made by rough machining from alloy steel stock, followed by heat treating and final machining. Satisfactory cutter heads can also be fabricated from castings of Ductile Iron, a close grain cast iron.

The cutter head 10 is usually assembled at a manufacturing facility and shipped to the field for use as a complete unit. In assembling the device, a blasting cap 26 is positioned in the bore 20 in firing head 11 and its lead wires connected, one to post 22 and the other to ground. An exact length of detonating fuse 46 is cut and a booster cap 59 is crimped on one end thereof. The length of fuse is inserted into the opening in-the top wall and cylindrical boss 30 of the cutter head and pushed in until the end hits the bottom of the bore 55 in the end plug 35. An O-ring 31 is placed around the boss 30 and the cutter head is then screwed into the firing head 11. When the threads are made up, the upper end of booster cap 59 butts against the lower end of the blasting cap 26 to provide a detonating train that may be fired from the earths surface. The firing head is connected to the adapter sub 12 and through it to the casing collar locator 13 and the conductor cable (not shown) by which the completely assembled apparatus is lowered into a string of tubing.

In operation, the cutter head is positioned in the tubing at the depth it is desired to sever the tubing. A switch (not shown) on the earths surface is closed to supply electric current to fire the blasting cap 26 which, in turn, fires the booster cap 59, detonating fuse 46 and cartridge 39. The shaped explosive charge cartridge detonates and forms a horizontal planar cutting jet traveling outwardly in the plane of the vertex of the lined shaped charge cavity. The jet severs the thin wall section 57 and continues outwardly to cut the surrounding tubing. 7

When the explosive sections 47-48 of the cartridge detonate, the side wall 33 is broken off transversely with little or no outward flaring at the apex of the V-shaped first annular groove 34. The side wall generally breaks along a line extending between the apex of the groove 34 and the point defined by the intersection of the surfaces of the counterbored interior side wall and the shoulder 50. The force of the explosion breaks the ring portion of the case lying between the first exterior annular groove 34 and second interior annular groove 56 into a number of separate pieces. As may be seen in FIG. 5, provision of the groove 34 in the exterior of the wall 33 prevents any slight irregular edge portions of the broken end of the cutter head from extending past the outline of the upper portion 33a of the side wall.

The force of the detonation of the cartridge also breaks the larger diameter, lower portion 33b of the side wall into several small pieces. The annular groove 53 and the axial bore 55 in the end plug provide lines of weakness to enhance break up of this portion of the device. The portion of the detonating fuse received in the bore 55 contributes substantially to break-up of the end plug. The end plug is preferably made'of a frangible material such as cast iron. Thus, it is seen that all of the portions of the cutter head below the first annular groove 34 are broken into pieces which are small enough to fall through the severed tubing without any danger of bridging in the tubing. The cutter head upper side wall portion 33a remains intact and substantially undistorted. The retrievable portions of the device, owing to their small diameter, are easily brought back through close fitting seating nipples that may be present in the tubing.

The embodiment of the invention shown in FIG. 1 is especially proportioned for severing 2 ,4 O.D., 0.156" wall, 1.721" I.D. tubing and is retrievable' back through an Otis S type seating nipple with a 1.625 ID. The CD. of the upper portion 331: of the side Wall is 1.442. The CD. of the lower portion 33b of the side Wall is 1.564". The height of the housing is 4 (without end plug) and the height of the chamber 40 is 2 measured from the bottom of the housing. The chamber 40 is A I.D. above the shoulder 50; the counterbo'red portion of the chamber which receives the cartridge is 1.093" ID. The lower portion 33b of the side wall is 1.050 in height. The side walls of groove 34 are illustrated as including a 60 angle but other angles may function just as satisfactorily. The apex of a the groove 34 defines a circle having a 1.317" diameter. The diameter of the groove 56 is 1.346" and the thin wall section 57 has a thickness of 0.050. The groove 53 and the bore 55 in the end cap 35 are both 0.318" deep. The cartridge 39 contains 4.0 grams of waxed RDX explosive with A of one percent graphite. The annular elements of the explosive are compressed with a force of 12 /2 tons.

From the foregoing description of the invention, it will be obvious to persons skilled in the art to make various modifications thereto and it is understood that,

such modifications are included within the spirit and scope of the invention as defined by the appended claims.

I claim: 1. A device for severing well tubing and the like comprising:

(a) a short cylindrical, pressure-resistant housing including a hollow body portion and a separate end plug sealingly closing the lower end of said body; (b) said body portion having a thick annular side wall and a thick top wall integral therewith; (c) said side wall providing a single first horizontal annular groove in the exterior surface of the lower portion thereof;

groove and said side wall having a thin section bound- V ing the outer periphery of said second groove;

(e) said body portion below said first annular groove having an outside diameter greater than the body portion above said first annular groove;

(f) a generally cylindrical, shaped explosive charge cartridge having a main charge of explosive material with an outwardly-opening, annular, lined cavity in the periphery mounted facing said thin section of said side wall and adapted upon being fired to produce a radially expanding planar cutting jet adapted to sever said sidewall at said thin section;

(g) said cartridge being positioned within said hollow body portion to place the upper surface. of said main charge of explosive material in a horizontal plane lying between said first and second annular grooves;

(h) means maintaining said cartridge in said position within said hollow body portion;

(i) said top wall and said cartridge providing axial openings therethrough adapted to receive a portion a of a detonating fuse; V V

(j) said housing and said cartridge defining therebetween a substantially unobstructed space above said cartridge providing an expansion chamber for upwardly acting forces produced upon detonation v of said cartridge;

(k) said body adapted to be cleanly broken transversely at said first annular groove by detonation of said cartridge whereby said body portion above said first annular groove will remain at substantially the same diameter as before detonation of said cartridge; and

1 (l) the side wall of said body above said first annular groove being of sufficient thickness and strength to remain intact and to be substantially undistorted after said cartridge is fired.

2. A device for severing well tubing and the prising: V

(a) a short cylindrical, pressure-resistant housing including a hollow body portion and a separate end plug sealingly closing the lower end of said body;

(b) said body portion having a thick annular side wall and'a thicktop wall integral therewith;

like com- V (c) said side wall providing a single first horizontal annular groove in the exterior surface of the lower portion thereof; (d) said side wall providing a second horizontal annular groove in the interior surface of said lower. portion thereof immediately below said first annulargroove and said side wall having a thin section bounding the outer periphery of said second groove; (e) said body portion below said first annular groove having an outside diameter greater than the body portion above said first annular groove;

(f) a generally cylindrical, shaped explosive charge cartridge having a main charge of explosive material with an outwardly-opening, annular, lined cavity in the periphery mounted facing said thin section of said side wall and adapted upon being fired to produce a radially expanding planar cutting jet adapted to sever said side wall at said thin section;

(g) said cartridge being positioned within said hollow V V body portion to place the upper surface'of said main between a substantially. unobstructed space above said cartridge providing an expansion chamber for upwardly acting forces produced upon detonation of said cartridge; 7

(k) said body adapted to be cleanly broken transversely at said first annular groove by detonation of said cartridge whereby said body portion above said first annular groove will remain at substantially the samediarneter as before detonation of said cartridge;

(1) the side wall of said body above said first annular groove being of sufficient thickness and strength to remain intact and to be substantially undistorted after said cartridge is fired; and

(m) said body portion below said first annular groove and said separate end plug being adapted to be broken into several pieces by detonation of said cartridge, and the end portion of a detonating fuse.

3. A device for severing well tubing and the like comprising:

(a) a short cylindrical,pressure-resistant housing including a hollow body portion and a separate end plug, sealingly closing the lower end of said body;

(b) said body portion having a thick'annular side wall and a thick top wall integral therewith;

(c) said side wall providing a single first horizontal annular groove in the exterior surface of the lower portion thereof;

(d) said side wall providing a second horizontal annular groove in the interior-surface of said lower portion thereof immediately below said first annular groove and said side wall having a thin section bounding the outer periphery of said second groove;

(c) said body portion below said first annular groove having an outside diameter-greater than the body portion above said first annular groove;

(f) a generally cylindrical, shaped explosive charge cartridge having a main charge of explosive material with an-outwardly-opening, annular lined cavity in the periphery mounted facing said thin section of said side wall and adapted upon being fired to produce a radially expanding planar cutting'jet adapted to sever said side wall atsaid thin section;

(g) said cartridge being positioned within said hollow body portion to place the upper surface of said main charge of explosive material in a horizontal plane lying between said first and second annular grooves;

(h) means maintaining said cartridge in said position within said hollow body portion;

(i) said top wall and said cartridge providing axial openings therethrough adapted to receive a portion of a detonating fuse;

(j) said housing and said cartridge defining therebetween a substantially unobstructed space above said cartridge providing an expansion chamber for 8 upwardly acting forces produced upon detonation of said cartridge;

(k) said bodyadapted to be cleanly broken transversely at said first annular groove by detonation of said ca rtridge whereby said body portion above said first annular groove will remain at substantially the same diameter as-before detonation of said cartridge;

' (l) the sidewall of said body above said first ancomprising:

'(a) a vertically elongated, hollow, pressure-resistant housing; -"(b) said housing having an upper cylindrical portion and a lower cylindrical portion of larger diameter than said upper cylindrical portion, said lower por- 'tion being" integral with and coaxial with said upper "portion, said housing providing a single exterior, annular groove defined between said upper-and lower portions, said groove being'coaxial with said upper and lower portions and having a diameter less than the diameter of either portion; (c) an annular, shaped explosive charge positioned in said housing coaxially with said housing portions and said groove, located immediately below said groove to project its jet circumferentially through the sidewall of said lower housing portion, and adapted upon being detonated to produce a clean break in said housing horizontally through said groove without producing any substantial increase in'the diameter of said upper cylindrical portion of said housing; and" (d) means for firing said shaped explosive charge.

hermetically sealed,

. References'Cited by the Examiner UNITED STATES PATENTS a FOREIGN PATENTS -1,098,104"' 3/55 France.

SAMUEL FEINBERG, .Primary Examiner. 

1. A DEVICE FOR SEVERING WELL TUBING AND THE LIKE COMPRISING: (A) A SHORT CYLINDRICAL, PRESSURE-RESISTANT HOUSING INCLUDING A HOLLOW BODY PORTION AND A SEPARATE END PLUG SEALINGLY CLOSING THE LOWER END OF SAID BODY; (B) SAID BODY PORTION HAVING A THICK ANNULAR SIDE WALL AND A THICK TOP WALL INTEGRAL THEREWITH; (C) SAID SIDE WALL PROVIDING A SINGLE FIRST HORIZONTAL ANNULAR GROOVE IN THE EXTERIOR SURFACE OF THE LOWER PORTION THEREOF; (D) SAID SIDE WALL PROVIDING A SECOND HORIZONTAL ANNULAR GROOVE IN THE INTERIOR SURFACE OF SAID LOWER PORTION THEREOF IMMEDIATELY BELOW SAID FIRST ANNULAR GROOVE AND SAID SIDE WALL HAVING A THIN SECTION BOUNDING THE OUTER PERIPHERY OF SAID SECOND GROOVE; (E) SAID BODY PORTION BELOW SAID FIRST ANNULAR GROOVE HAVING AN OUTSIDE DIAMETER GREATER THAN THE BODY PORTION ABOVE SAID FIRST ANNULAR GROOVE; (F) A GENERALLY CYLINDRICAL, SHAPED EXPLOSIVE CHARGE CARTRIDGE HAVING A MAIN CHARGE OF EXPLOSIVE MATERIAL WITH AN OUTWARDLY-OPENING, ANNULAR, LINED CAVITY IN THE PERIPHERY MOUNTED FACING SAID THIN SECTION OF SAID SIDE WALL AND ADAPTED UPON BEING FIRED TO PRODUCE A RADIALLY EXPANDING PLANAR CUTTING JET ADAPTED TO SEVER SAID SIDE WALL AT SAID THIN SECTION; (G) SAID CARTRIDGE BEING POSITIONED WITHIN SAID HOLLOW BODY PORTION TO PLACE THE UPPER SURFACE OF SAID MAIN CHARGE OF EXPLOSIVE MATERIAL IN A HORIZONTAL PLANE LYING BETWEEN SAID FIRST AND SECOND ANNULAR GROOVES; (H) MEANS MAINTAINING SAID CARTRIDGE IN SAID POSITION WITHIN SAID HOLLOW BODY PORTION; (I) SAID TOP WALL AND SAID CARTRIDGE PROVIDING AXIAL OPENINGS THERETHROUGH ADAPTED TO RECEIVE A PORTION OF A DETONATING FUSE; (J) SAID HOUSING AND SAID CARTRIDGE DEFINING THEREBETWEEN A SUBSTANTIALLY UNOBSTRUCTED SPACE ABOVE SAID CARTRIDGE PROVIDING AN EXPANSION CHAMBER FOR UPWARDLY ACTING FORCES PRODUCED UPON DETONATION OF SAID CARTRIDGE; (K) SAID BODY ADAPTED TO BE CLEANLY BROKEN TRANSVERSELY AT SAID FIRST ANNULAR GROOVE BY DETONATION OF SAID CARTRIDGE WHEREBY SAID BODY PORTION ABOVE SAID FIRST ANNULAR GROOVE WILL REMAIN AT SUBSTANTIALLY THE SAME DIAMETER AS BEFORE DETONATION OF SAID CARTRIDGE; AND (L) THE SIDE WALL OF SAID BODY ABOVE SAID FIRST ANNULAR GROOVE BEING OF SUFFICIENT THICKNESS AND STRENGTH TO REMAIN INTACT AND TO BE SUBSTANTIALLY UNDISTORTED AFTER SAID CARTRIDGE IS FIRED. 